Sew-in interfacing should be preshrunk by the same method of care that will be used for the finished garment. Dry-clean only interfacings can by preshrunk by steaming with a steam iron and letting it air dry before use.
To preshrink fusible interfacing, place loosely folded interfacing in a tub of hot water and let it soak until the water is cool. Carefully squeeze out excess water by gently pressing it between two towels; do not wring. Drape over a shower rod or towel rack with the fusible resin side up and let air dry.